docs: def setSamplefrequency(self, edfsignal, samplefrequency):
          """
          Sets the samplefrequency of signal edfsignal.
  
          Notes
          -----
          This function is required for every signal and can be called only after opening a file in writemode and before the first sample write action.
          """
          if edfsignal < 0 or edfsignal > self.n_channels:
              raise ChannelDoesNotExist(edfsignal)
          self.channels[edfsignal]['sample_rate'] = samplefrequency
          self.update_header()